Determinants and outcomes of group coordination and decision-making in red-fronted lemurs (Eulemur rufifrons)

Abstract

In order to reap the benefits of group-living, gregarious animals have to maintain group cohesion. However, differences among group members in terms of sex, age or physiological constitution lead to different activity budgets and motivations, which, in turn, act as a centrifugal force on group cohesion. In order to stay cohesive despite inter-individual conflicts, groups must coordinate collective actions by making shared or unshared consensus decisions. In this context, single group members often adopt different behavioural roles, i.e. certain individuals emerge as leaders, others as followers. In the last decade, a number of studies explored single aspects of coordinated behaviour in the context of group movements, covering a wide range of taxa. Yet, comparisons among studies are still hampered by ambiguous terminology and inconsistent concepts. In addition, taxon- and condition-specific determinants of coordination processes and decision-making under natural conditions remain poorly understood. I addressed both of these general issues in my thesis. First, I outlined why group movements emerged as the major paradigm of coordination research and why primates provide an excellent model taxon in this context, followed by a review of several aspects related to primate group movements. With this basis, I shifted the focus towards terminology, methodology and concepts used in (primate) coordination research and made suggestions to improve comparability among future studies. In particular, I encouraged the use of taxon-specific definitions of group movements and a clear differentiation between coordination research on the individual-level process (decision-making) and the group-level outcome (leadership). In a second part of the thesis, I explored group movements in wild red-fronted lemurs (Eulemur rufifrons) with a comprehensive approach that included (i) analyses of spatio-temporal patterns of ranging behaviour using GPS collars, (ii) observations of coordination processes based on an operational definition in four groups across naturally variable socio-ecological contexts, and (iii) observations and video recordings of coordination and decision outcomes in one group during a field experiment aimed at manipulating the cost/benefit ratio of individual versus group-level decisions through the use of drinking platforms. Analyses of GPS ranging data revealed strong impacts of analytical method, sampling regime and seasonality on home range estimates. Under natural conditions, stable leadership by adult females prevailed year-round, presumably reinforced by close affiliative network relations among philopatric females. However, adult male initiators had the same recruitment success as females supposedly a consequence of the species` egalitarian social structure. Recruitment success was higher during the lean season, possibly due to a higher predation risk. Adult females also initiated most movements during the experiment. However, leaders were overtaken in 67% of observed cases. Consequently, individual resource intake at the foraging platforms did not depend on sex, age or departure position but only on arrival order. Moreover, group fission occurred in 11% of cases, which resulted in more individuals drinking simultaneously but also temporarily low cohesion. The type of decision-making (shared or unshared) could not be determined because lemur visits at the platforms during the various patterns of baiting (monopolisable versus non-monopolisable resources) did not meet our predictions, but rather occurred randomly. In conclusion, these results suggest that coordination processes are highly flexible, depending on environmental variations, such as seasonality and resource quality/predictability. Furthermore, adult recruitment success seems to be equally distributed among sexes, and group members may use alternative behavioural tactics to consensus decisions, such as overtaking or fissioning, in order to optimise individual foraging benefits irrespective of sex, age or social status. The findings in red-fronted lemurs might be representative of gregarious species characterised by egalitarian social relationships.